Diffraction grating coupled laser ring resonators

ABSTRACT

In the disclosed laser ring resonators a pair of mirrors and at least one diffraction grating are disposed relative to an active laser medium, such as CO2, such that laser energy generated in the medium traverses a unidirectional closed circulation path through the medium. The diffraction grating has a groove spacing d related to the laser energy wavelength  lambda  by 1 &lt;  lambda /d &lt; 2 and diffracts a portion of the incident laser energy along the circulation path and another portion out of the circulation path. A wavelength selecting embodiment utilizes two diffraction gratings in the circulation path through the laser medium, while a wavelength selecting and axial mode selecting embodiment further employs an additional mirror to provide an axial mode selecting closed optical path partially coincident with a wavelength selecting closed optical path through the laser medium.

United States Patent Janney 1 Se t. 12, 1972 [54] DIFFRACTION GRATINGCOUPLED LASER RING RESONATORS [72] Inventor: Gareth M. Janney, PacificPalisades,

Calif.

[73] Assignee: Hughes-Aircraft Culver City, Calif.

22 Filed: Sept.4, 1970 211 Appl. No.: 78,323

Company,

Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGrawAttorney-James K. Haskell and Paul M. Coble [57] ABSTRACT In thedisclosed laser ring resonators a pair of mirrors and at least onediffraction grating are disposed relative to an active laser medium,such as C0,, such that laser energy generated in the medium traverses aunidirectional closed circulation path through the medium. Thediffraction grating has a groove spacing d related to the laser energywavelength A by l A/d 2 and diffracts a portion of the incident laserenergy along the circulation path and another portion out of thecirculation path. A wavelength selecting embodiment utilizes twodiffraction gratings in the circulation path through the laser medium,while a wavelength selecting and axial mode selecting embodiment furtheremploys an additional mirror to provide an axial mode selecting closedoptical path partially coincident with a wavelength selecting closedoptical path through the laser medium.

10 Claims, 4 Drawing Figures DIFFRACTION GRATING COUPLED LASER RINGRESONATORS The invention herein described was made in the course of orunder a Contract or Subcontract thereunder with the United States AirForce.

This invention relates to lasers, and more particularly relates to laserring resonators employing one or more diffraction gratings to coupleenergy out of the resonator.

In a laser ring resonator at least three laser energy reflectingelements are disposed relative to an active laser medium so as to causelaser energy generated in the medium to traverse a unidirectional closedcirculation path through the medium. In one type of laser ring resonatoraccording to the prior art, energy is coupled out of the resonator bymeans of a beam splitter which also functions as one of the energyreflecting elements. When beam splitter coupled laser ring resonatorsare operated at relatively high average power levels, e.g. greater thanaround a kilowatt, the beam splitter material becomes significantlyabsorptive at infrared wavelengths. This produces excessive heatingwhich may result in destruction of the beam splitter coating and/orfracture of the beam splitter itself.

Another prior art laser ring resonator arrangement utilizes apertures inthe reflecting elements to couple laser energy out of the resonator.This type of resonator produces laser beams of non-uniformcross-sectional area or intensity distribution, resulting in reducedeffi ciency when hollow beams traverse the laser medium. Moreover, it isvery difficult to interferometrically recombine a plurality of suchnon-uniform circulating laser beams so as to produce a useful output.

Accordingly, it is an object of the present invention to provide a laserring resonator which is operable at higher average power levels thanlaser ring resonators of the prior art, especially at infraredwavelengths.

It is a further object of the invention to provide a high power laserring resonator in which laser energy is readily removed from theresonator without significantly altering the cross-sectional area orintensity distribution of the circulating laser beam.

It is a still further objectof the invention to provide a laser ringresonator which additionally limits travelingwave oscillation to awavelength corresponding to a preselected laser transition in the activelaser medium.

It is yet another object of the invention to provide a laser ringresonator in which traveling-wave oscillation is limited to both apreselected wavelength and a single axial mode. I

In a laser ring resonator according to the invention an active lasermedium, at least two optical energy reflecting devices, and adiffraction grating are disposed along a closed optical path through thelaser medium and are oriented such that laser energy generated in themedium circulates unidirectionally along the closed optical path. Thediffraction grating has a groove spacing such that a portion of thelaserenergy traveling along the closed optical path and incident on thegrating is diffracted by the grating along the closed optical path,while another portion of the laser energy traveling along the closedpath and incident on the grating is diffracted by the grating out of theclosed optical path.

apparent from the following detailed description of the invention whenconsidered in conjunction with the accompanying drawings in which:

FIG. 1 schematically illustrates a diffraction grating coupled laserring resonator in accordance with one embodiment of the invention;

FIG. 2 is a diagram showing beam splitting properties of a typicaldiffraction grating which may be employed in a ring resonator accordingto the invention;

FIG. 3 schematically illustrates a wavelength selecting diffractiongrating coupled laser ring resonator in accordance with anotherembodiment of the invention; and

FIG. 4 is a schematic illustration of a wavelength and axial modeselecting diffraction grating coupled laser ring resonator in accordancewith still another embodiment of the invention. i

Referring to FIG. 1 with greater particularity, a diffraction gratingcoupled laser ring resonator according to the invention may be seen toinclude an active laser medium 10 which may be pumped to a lasing stateby a pump source (not shown). An example of a particular laser medium 10which may be employed is CO operating to produce output laser energy atessentially 10.6 2, and in the ensuing discussion specific exemplarygrating design parameters are given for this particular laser. However,it should-be understood that other laser media including but not limitedto ruby, Nd-YAG, helium-neon, and argon, for example, may be employedwithin the principles of the invention, although the invention isespecially useful with lasersoperating at infrared wavelengths.

A pair of optical energy reflecting devices, illustrated as mirrors l2and 14, and a diffraction grating 16 are disposed at respectivelocations relative to the medium 10 and are oriented so as to reflectlaser energy generated in the medium 10 into a unidirectional closedoptical circulation path through the medium 10. The mirrors 12 and 14are selected to have maximum reflectivity in the vicinity of the laserwavelength of interest (l0.6p. in the case of a C0 laser medium), andmay be of polished metal with an aluminum, silver, or gold coating, forexample. The diffraction grating 16, which may also be of polished metalwith an aluminum, silver, or gold coating, is provided with a series ofparallel grooves of uniform spacing d. The angle between the groovesurface and the normal to the plane of the grating is referred to as theblaze angle.

In order to better understand the function and operation of thediffraction grating 16, reference may be made to FIG. 2 whichillustrates the beam splitting properties of a typical diffractiongrating. A beam of radiation incident on the diffraction grating at anangle 1 with respect to the normal to the grating is diffracted by thegrating into n beams in accordance with the 'relation I l )\/d 2 2 thenonly two diffracted beams will be produced corresponding to the zeroorder (n O) and the first order (n l) for a given angle of incidence IThis condition is illustrated in FIG. 2 where the incident beam i isshown diffracted into the zero order and first order beams designated nand n 1, respectively.

In laser ring resonators according to the present invention thediffraction gratings are designed so that only the zero order and firstorder diffracted beams are produced. For example, for an active lasermedium of C0 producing radiation at a wavelength A l0.6p., a gratinghaving 150 grooves per mm may be used. In this case the grating spacingd 6.7g, and M11 1.59. Using equation (1) it may be determined that for aselected angle of incidence I of 45, for example, the first order (n l)diffracted beam will have an angle of diffraction 0 of 62. The zeroorder diffracted beam (n 0) will be diffracted at an angle equal inmagnitude but opposite in sign to the angle of incidence 1 Thus, as maybe seen from FIG. 1, the laser beam 18 which is incident upondiffraction grating 16 is diffracted by the grating 16 into a zero order(n 0) diffracted beam 20 and a first order (n l) diffracted beam 22. Thebeam 20 is directed back into the laser medium 10 along the closedoptical circulation path via the mirror 14, while the beam 22constitutes the output beam from the resonator and is directed out ofthe optical circulation path.

The relative amounts of optical power which are diffracted into the n 0and n 1 orders is afunction of the blaze angle of the grating grooves,the angle of incidence of the radiation on the grating, and thepolarization of the incident radiation. When 10.6p. radiation isincident upon an aluminum coated grating having 150 grooves per mm atthe aforementioned angle of incidence (I of 45 (or on account ofreciprocal properties of the grating at the aforementioned diffractionangle 0 of 62) with its electric field vector E polarized perpendicularto the length of the grating grooves, the relative mounts of powerdiffracted into the n 0 and the n 1 orders for several grating blazeangles is essentially as follows:

A small amount of power on the order of 2-3 percent is lost due toabsorption and scattering.

In order to sustain oscillations, a C0 laser with an active mediumlength of one meter requires that at least about 67 percent of itsemitted laser energy be fed back into the active medium. Thus, it may beseen from Table I that when such a laser is used as the active medium 10in a ring resonator according to FIG. 1 with the aforementioned specificdesign parameters and with the n 0 order diffracted beam recirculatingthrough the laser medium, a blaze angle of essentially between 2 and 10would be appropriate for the grating 16.

In a further embodiment of the invention, illustrated in FIG. 3, adiffraction grating coupled laser ring resonator is provided whichselectively limits travelingwave oscillation to a wavelengthcorresponding to a preselected laser transition (atomic or molecular) inthe active laser medium. As shown in FIG. 3, active laser medium-30 iscoupled in a ring resonator arrangement with mirrors 32 and 34 and apair of diffraction gratings 36 and 38 which are disposed so as toprovide a unidirectional closed circulation path for laser energythrough the active medium 30. For an active medium 30 of CO theaforementioned aluminum coated diffraction grating having grooves per mmmay be used for each of the gratings 36 and 38, for example, althoughother media and gratings are equally suitable.

In the particular embodiment illustrated in FIG. 3, laser beam 40 whichis incident upon grating 36 is dif fracted into a zero order (n 0) beam42 and a first order (n l) beam 44. The beam 42'constitutes an outputbeam from the ring resonator, while the beam 44 is directed along thelaser energy circulation path. Beam 44 is reflected by mirror 34 ontodiffraction grating 38 which diffracts this beam into a zero order (n 0)beam 46 and a first order (n l) beam 48. The beam 46 is directed out ofthe ring resonator and may be used as an auxiliary output beam, whilethe beam 48 is fed back through the laser medium 30.

In order to insure that the amount of laser energy returned to themedium 30 is sufficient to sustain oscillations, the blaze angles of thegratings 36 and 38 may be determined in the manner discussed above withrespect to Table 1. Thus, as an example, when employing a C0 lasermedium of one meter active length, alu-' minum coated diffractiongratings having 150 grooves per mm and angles of grating incidence anddiffraction of 45 or 62 in the arrangement of FIG. 2, grating 36 mayhave a blaze angle of 17 and grating 38 a blaze angle of 37. When theseblaze angle values are used, approximately 68 percent of the laserenergy will be returned to the medium 30.

Although the foregoing discussion has considered the laser medium asoperating at a single wavelength (e.g. l0.6,u.), laser media usuallyprovide several transitions in the immediate vicinity of the specifiedwavelength. For example, while a C0 laser is norm ally specified to havean output wavelength of 10.6,u, a plurality of different transitionsactually provide a plurality of output wavelengths ranging essentiallyfrom 9.711. to 10.8u. From Equation l it will be apparent that for afixed angle of incidence on a diffraction grating, the angle at whichthe first order beam will be diffracted varies as a function of theradiation wavelength. Thus, when the active medium 30 of the ringresonator of FIG. 3 generates laser energy at slightly differentwavelengths, these different wavelengths will be diffracted by thegratings 36 and 38 into slightly different paths, and laser energy atonly a particular wavelength will be recirculated through the medium 30.The ring resonator arrangement of FIG. 3 thus functions as a laserwavelength selector which limits traveling-wave oscillation to awavelength corresponding to a preselected transition in the active lasermedium 30. The two-grating ring resonator of FIG. 3 possesses anadvantage over a single-grating arrangement according to FIG. 1 whereinthe first order beam is circulated through the medium in that changes inthe cross-section of the beam upon diffraction by one of the gratingsare compensated for by equal and opposite changes upon diffraction bythe other grating.

In a still further embodiment of the invention, illustrated in FIG. 4, alaser ring resonator is provided which affords both laser wavelengthselection and axial mode selection. In the embodiment of FIG. 4 activelaser medium 50 is coupled in a ring resonator arrangement with mirror52, diffraction grating 56, mirror 54, and diffraction grating 58. Theelements 52, 54, 56 and 58 are similar to respective elements 32, 34, 36and 38 of the resonator of FIG. 3 and function as a primary, orwavelength selecting, resonator 55 which provides a unidirectionalclosed circulation path for laser energy through the active medium 50.The operation of the primary resonator 55 of FIG. 4 is the same as thatof the ring resonator of FIG. 3, and laser beam paths in FIG. 4 whichare equivalent to corresponding paths in FIG. 3 are shown in solid linein FIG. 4. Thus, beam 60 of FIG. 4 corresponds to beam 40 of FIG. 3;beam 64 to beam 44; beam 68 to beam 48; and beam 62 to beam 42.

In the embodiment of FIG. 4 an additional mirror 70 is disposedoptically between diffraction gratings 56 and 58 so as to form inconjunction with mirror 54 and gratings 56 and 58 a secondary, or axialmode selecting, resonator 71 providing a closed optical energycirculation path shown in dashed line in FIG. 4. As may be seen fromFIG. 4, the portion of the optical path of secondary resonator 71between the diffraction gratings 56 and 58 via the mirror 54 iscoincident with that of primary resonator 55. However, the zero orderbeam 72 diffracted by grating 58 (which corresponds to beam 46 of FIG.3) is reflected by mirror 70 onto diffraction grating 56. The beam 72is, in turn, diffracted by grating 56 into a zero order beam 74 and afirst order beam 76. The mirror 70 is disposed so as to direct beam 72onto the grating 56 at an angle of incidence such that the diffractedbeam 76 is superimposed on the diffracted beam 62, resulting ininterferometric combination of the beams 76 and 62 into a compositeoutput beam. The diffracted beam 74, which is superimposed on diffractedbeam 64, is reflected by mirror 54 onto grating 58. The beam 74 is, inturn, diffracted by grating 58 into the zero order diffracted beam 72and a first order diffracted beam 78 which, along with beam 68, is fedback to the laser medium 50.

As was explained above with respect to the ring resonator of FIG. 3, theprimary resonator 55 of FIG. 4 is designed to limit oscillation to awavelength corresponding to a particular atomic or molecular transitionin the laser medium 50. The energy circulation path length L around thesecondary resonator 71 may be selected to limit oscillation to a singleaxial mode for the laser transition selected by the primary resonator55. This may be achieved by making the path length L such that thefrequency separation Av c/L (where c is the velocity of light) betweenadjacent oscillatory modes of the resonator 71 is greater than the widthof the atomic or molecular gain curve for the selected laser transition.For example, for transitions in most CO lasers a secondary resonatorpath length L of 4 meters or less will afford the desired single axialmode operation.

The particular gratings 56 and 58 used in the ring resonator of FIG. 4should provide sufficient feedback of laser energy to the medium 50 tosustain oscillations. As an example, for a C0 laser medium of a onemeter active length used with aluminum coated diffraction gratingshaving grooves per mm in a resonator arrangement according to FIG. 4 inwhich the angles of grating incidence and diffraction are 45 or 62, ablaze angle of 17 for the grating 56 and a blaze angle of 10 for thegrating 58 should afford ample feedback to insure that oscillations willbe sustained.

It is further pointed out that while the laser energy has been shown anddescribed as circulating around the ring resonators of FIGS. 1, 3 and 4in the clockwise sense, counterclockwise circulation is also possible.Circulation in the desired sense only may be insured by including anonreciprocal optical element such as a Faraday isolator in the closedoptical path.

Although the present invention has been shown and described withreference to particular embodiments, nevertheless, various changes andmodifications obvious to one skilled in the art to which the inventionpertains are deemed to lie within the-spirit, scope and contemplation ofthe invention.

What is claimed is:

1. A laser ring resonator comprising:

an active laser medium, at least two optical energy reflecting devicesand a diffraction grating disposed along a closed optical path throughsaid laser medium and oriented such that laser energy generated in saidmedium circulates unidirectionally along said path, said diffractiongrating having a groove spacing such that a first portion of the laserenergy traveling along said path and incident on said grating isdiffracted by said grating along said path and a second portion of saidtraveling incident laser energy is diffracted by said grating out ofsaid path.

2. A laser ring resonator according to claim 1 wherein the ratio of thewavelength of said circulating laser energy to said diffraction gratinggroove spacing is of a numerical value between 1 and 2.

3. A laser ring resonator according to claim 2 wherein said diffractiongrating is oriented such that said first portion of laser energy isdiffracted by said grating along the direction of the zero order of saidgrating and said second portion of laser energy is diffracted by saidgrating along the direction of the first order of said grating.

4. A laser ring resonator comprising:

an active laser medium; a first optical energy reflecting device, afirst diffraction grating, a second optical energy reflecting device,and a second diffraction grating successively disposed along a closedoptical path through said laser medium and oriented such that laserenergy generated in said medium circulates unidirectionally along saidpath; at least one of said first and second diffraction gratings havinga groove spacing such that a first portion of the laser energy travelingalong said path and incident on said one grating is diffracted by saidone grating along said path and a second portion of said travelingincident laser energy is diffracted by said one grating out of saidpath.

5. A laser ring resonator according to claim 4 wherein the ratio of thewavelength of said circulating laser energy to said diffraction gratinggroove spacing is of a numerical value between one and two.

6. A laser ring resonator according to claim 5 wherein the other of saidfirst and second diffraction gratings has a groove spacing such that afirst portion of the laser energy traveling along said path and incidenton said other grating is diffracted by said other grating along saidpath and a second portion of said traveling laser energy incident onsaid other grating is diffracted by said other grating out of said path.

7. A laser ring resonator comprising:

an active laser medium; a first optical energy reflecting device, afirst diffraction grating, a second optical energy reflecting device,and a second diffraction grating successively disposed along a firstclosed optical path through said laser medium and oriented such thatlaser energy generated in said medium circulates unidirectionally alongsaid first path; a third optical energy reflecting device disposedoptically between said first and second diffraction gratings along asecond closed optical path having a portion coincident with the portionof said first optical path extending between said first and seconddiffraction gratings via said second optical energy reflecting device;said first diffraction grating having a groove spacing and orientationsuch that a first portion of the laser energy traveling along said firstpath and incident on said first grating is diffracted by said firstgrating into a selected path out of said first and second paths and asecond portion of the laser energy traveling along'said first path andincident on said first grating is diffracted by said first grating alongsaid coincident portion of said first and second paths, while a firstportion of the laser energy traveling along said second path andincident on said first grating is diffracted by said first grating alongsaid coincident portion of said first and second paths, and a secondportion of the laser energy traveling along said second path andincident on said first grating is diffracted by said first grating intosaid selected path; said second grating having a groove spacing andorientation such that a first portion of the laser energy travelingalong said coincident portion of said first and second paths andincident onsaid second grating is diffracted by said second gratingalong said second path, and a second portion of the laser energytraveling along said coincident portion of said first and second pathsand incident on said second grating is diffracted by said second gratingalong said first path. 8. A aser ring resonator according to claim 7wherein the ratio of the wavelength of said circulating laser energy tothe groove spacing of each of said first and second diffraction gratingsis of a numerical value between one and two.

9. A laser ring resonator according to claim 8 wherein said firstdiffraction grating is oriented such that said first portion of laserenergy diffracted by said first grating from said first path isdiffracted along the direction of the zero order of said first grating,said second portion of laser energy diffracted by said first gratingfrom said first path is diffracted along the direction of the firstorder of said first grating, said first portion of laser energydiffracted by said first grating from said second path is diffractedalong the direction of the zero order of said first grating, and saidsecond portion of laser energy diffracted by said first grating fromsaid second path is diffracted along the direction of the first order ofsaid first grating; said second grating being oriented such that saidfirst portion of laser energy diffracted by said second grating isdiffracted along the direction of the zero order of said second grating,and said second portion of laser energy diffracted by said secondgrating is diffracted along the direction of the first order of saidsecond grating.

10. A laser ring resonator according to claim 7 wherein the length ofsaid second closed optical path is of a selected value permitting only asingle axial mode of laser oscillation.

1. A laser ring resonator comprising: an active laser medium, at leasttwo optical energy reflecting devices and a diffraction grating disposedalong a closed optical path through said laser medium and oriented suchthat laser energy generated in said medium circulates unidirectionallyalong said path, said diffraction grating having a groove spacing suchthat a first portion of the laser energy traveling along said path andincident on said grating is diffracted by said grating along said pathand a second portion of said traveling incident laser energy isdiffracted by said grating out of said path.
 2. A laser ring resonatoraccording to claim 1 wherein the ratio of the wavelength of saidcirculating laser energy to said diffraction grating groove spacing isof a numerical value between 1 and
 2. 3. A laser ring resonatoraccording to claim 2 wherein said diffraction grating is oriented suchthat said first portion of laser energy is diffracted by said gratingalong the direction of the zero order of said grating and said secondportion of laser energy is diffracted by said grating along thedirection of the first order of said grating.
 4. A laser ring resonatorcomprising: an active laser medium; a first optical energy reflectingdevice, a first diffraction grating, a second optical energy reflectingdevice, and a second diffraction grating successively disposed along aclosed optical path through said laser medium and oriented such thatlaser energy generated in said medium circulates unidirectionally alongsaid path; at least one of said first and second diffraction gratingshaving a groove spacing such that a first portion of the laser energytraveling along said path and incident on said one grating is diffractedby said one grating along said path and a second portion of saidtraveling incident laser energy is diffracted by said one grating out ofsaid path.
 5. A laser ring resonator according to claim 4 wherein theratio of the wavelength of said circulating laser energy to saiddiffraction grating groove spacing is of a numerical value between oneand two.
 6. A laser ring resonator according to claim 5 wherein theother of said first and second diffraction gratings has a groove spacingsuch that a first portion of the laser energy traveling along said pathand incident on said other grating is diffracted by said other gratingalong said path and a second portion of said traveling laser energyincident on said other grating is diffracted by said other grating outof said path.
 7. A laser ring resonator comprising: an active lasermedium; a first optical energy reflecting device, a first diffractiongrating, a second optical energy reflecting device, and a seconddiffraction grating successively disposed along a first closed opticalpath through said laser medium and oriented such that laser energygenerated in said medium circulates unidirectionally along said firstpath; a third optical energy reflecting device disposed opticallybetween said first and second diffraction gratings along a second closedoptical path having a portion coincident with the portion of said firstoptical path extending between said first and second diffractiongratings via said second optical energy reflecting device; said firstdiffraction grating having a groove spacing and orIentation such that afirst portion of the laser energy traveling along said first path andincident on said first grating is diffracted by said first grating intoa selected path out of said first and second paths and a second portionof the laser energy traveling along said first path and incident on saidfirst grating is diffracted by said first grating along said coincidentportion of said first and second paths, while a first portion of thelaser energy traveling along said second path and incident on said firstgrating is diffracted by said first grating along said coincidentportion of said first and second paths, and a second portion of thelaser energy traveling along said second path and incident on said firstgrating is diffracted by said first grating into said selected path;said second grating having a groove spacing and orientation such that afirst portion of the laser energy traveling along said coincidentportion of said first and second paths and incident on said secondgrating is diffracted by said second grating along said second path, anda second portion of the laser energy traveling along said coincidentportion of said first and second paths and incident on said secondgrating is diffracted by said second grating along said first path.
 8. Alaser ring resonator according to claim 7 wherein the ratio of thewavelength of said circulating laser energy to the groove spacing ofeach of said first and second diffraction gratings is of a numericalvalue between one and two.
 9. A laser ring resonator according to claim8 wherein said first diffraction grating is oriented such that saidfirst portion of laser energy diffracted by said first grating from saidfirst path is diffracted along the direction of the zero order of saidfirst grating, said second portion of laser energy diffracted by saidfirst grating from said first path is diffracted along the direction ofthe first order of said first grating, said first portion of laserenergy diffracted by said first grating from said second path isdiffracted along the direction of the zero order of said first grating,and said second portion of laser energy diffracted by said first gratingfrom said second path is diffracted along the direction of the firstorder of said first grating; said second grating being oriented suchthat said first portion of laser energy diffracted by said secondgrating is diffracted along the direction of the zero order of saidsecond grating, and said second portion of laser energy diffracted bysaid second grating is diffracted along the direction of the first orderof said second grating.
 10. A laser ring resonator according to claim 7wherein the length of said second closed optical path is of a selectedvalue permitting only a single axial mode of laser oscillation.